Accelerometers are useful in many applications. In one application, as in airbag inflation in automobiles, a linear acceleration sensor system provides a signal to initiate inflation of the airbag in response to acceleration above a given threshold in a front or rear collision. In general, acceleration, being the second derivative of displacement, is an advance indicator of an impending change in position or displacement of an object and provides a signal sufficiently in advance of velocity build up and displacement of an object, to provide lead time in taking action to restrain and limit object displacement. In a vehicle, this limits displacement of the occupant in contact with the airbag and distributes the restraining force over a large area of the occupants body.
Another application is in a closed loop control system for moving and positioning an object, in which an acceleration force, due to mechanical shock, for example, has a force component acting in a direction to displace the object from a command position. The accelerometer in the system produces a control signal proportional to the acceleration. This acceleration signal is coupled in the control loop in a sense to apply a force to the object in a direction which, ideally, is opposite to and equal to the force acting on the object due to the mechanical shock, to thereby prevent or minimize displacement of the object from command position.
Accelerometers are advantageously employed in disk drives. Here, closed loop control systems are employed in driving the actuators which move the transducers to different radial positions on a disk in a track seeking mode of operation and, when a target track is found, to utilize the transducer in a closed loop, track following mode of operation to maintain the transducer in a track centered position, for recording or reading purposes.
Linear accelerometers are useful in both linear actuator drives and unbalanced rotary actuator drives for sensing acceleration forces acting along the path of movement of the mass center of the actuator. Signals from the accelerometer, when coupled into the closed loop and properly calibrated, produce forces acting on the actuator which minimize displacement of the transducer from the target track.
Trends in disk drives are to pack higher densities of information recordings into drives of smaller form factor, that is, smaller packages. This reduces available space on the circuit board for accommodating the control system for a drive, indicating the need for an accelerometer compatible with an integrated circuit environment.
Accelerometers for use in airbag inflation control in automobiles include an integrated circuit compatible accelerometer fabricated by surface micromaching techniques. Such an accelerometer is described by Frank Goodenough in an article entitled "Airbags Boom When IC Accelerometer Sees 50 G", Electronic Design, Aug. 8, 1991. This IC accelerometer is micromachined from polysilicon on a monocrystaline-silicon substrate. It is a variable differential capacitor type of linear acceleration sensor having fixed and movable capacitor electrodes or plates, the latter comprising part of a movable, tethered mass which is supported to be displaced relative to the fixed plates in response to linear acceleration.
Such a linear accelerometer has potential for application in disk drives employing linear actuators or rotary actuators that are not mass balanced, but is not suited for application in a disk drive which has a mass balanced rotary actuator which is relatively insensitive to linear acceleration and requires no compensation in that respect.